The invention herein relates to precision electromagnetic radiation intensity comparators and, more particularly, to monolithic integrated circuit electromagnetic radiation comparators for use in cameras or other systems with electromagnetic radiation input signals. In these uses the integrated circuit may also, of course, contain further signal processing circuitry.
There are a number of system situations in which it is required that differences in electromagnetic radiation intensity occurring at two or more sources be determined as a system input. For instance, a camera which is to use incident light from a scene desired to be photographed to adjust the camera focus requires, in one approach, that a correlation be performed between two views of the scene desired to be photographed. Local differences in light intensity in the images corresponding to these two views are indicative of how far from focus that camera currently is and so signals obtained from these images that are related to these differences can be used to automatically bring the camera into focus. In another instance, system electrical signals from various sources in the system can be sensed through optical couplers to provide nearly perfect isolation between the signal sources and the sensing portion of the system. Differences between the intensity of these signals as coupled through photodetectors will, in many instances, be of substantial interest in determining or furthering one or more system functions.
There are substantial difficulties in obtaining an indication of such electromagnetic intensity differences, particularly when the indication must be precise. One of the reasons for this is the rather wide range of electromagnetic radiation intensities often encountered by such difference determining systems, i.e. comparator systems. A camera system, typically, will encounter light conditions ranging over several orders of magnitude in intensity. To obtain precise differences at low light levels requires substantial amplification while carefully avoiding any noise pick up in the signal system. Yet at high light levels, the system must again provide a precise indication of differences, at least where the differences are not too great, without saturating the measuring system. Further, such measuring systems must often operate over a wide range of environmental conditions in obtaining the desired measurements and so must be able to provide precise difference indications throughout this range of conditions.
Electromagnetic radiation intensity comparators, particularly for light, have been provided heretofore but usually using discrete or partially monolithic integrated circuit techniques. Such embodiments, however, are subject to noise signal pick-up for low level electromagnetic radiation inputs and to thermal mismatch problems which cause similarly situated comparator system components to perform differently from one another. These lead to errors which are too large to be tolerated in a precision system where differences between various light intensities must be known to a very few percentage points of the light intensities being sensed and over a wide range of light intensities and operating conditions.